Add asterisks to required fields

[KarolDawidziuk]

What is the purpose of this pull request?

New feature

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#3433>](/~https://github.com/ckeditor/ckeditor4/issues/3433): [dialog]: Added asterisks to required fields.

What changes did you make?

I've added asterisks to each required label in the dialogs.

Which issues does your PR resolve?

Closes #3433.

[jacekbogdanski]

Please, add a manual test confirming that the feature works correctly for different editor skins (required once you move inlined CSS). You can just use some existing feature dialog like image, table, etc

[KarolDawidziuk]

Thanks @jacekbogdanski for the review!
It's ready for another one.

Changes:

• Added custom CSS class instead of inline style.
• Updated unit tests.
• Added manual tests.

[jacekbogdanski]

Good job 👍🏻